This invention relates to a continuous process for the production of d,l-menthol, and to the catalyst used in this process.
As it is known d,l-menthol, in the form produces in the examples herof, is useful for example in perfumes.
It is known that d,l-methol can be obtained by the batch catalytic hydrogenation of thymol and other compounds which have the carbon skeleton of menthol with at least one double bond and which are substituted by oxygen in the 4-position. It is also possible to obtain d,l-menthol by racemising and/or rearranging d-menthol and the optically active or inactive stereoisomers of menthol, for example by heating these compounds with hydrogen in the presence of a hydrogenation catalyst. A crude product containing d,l-menthol d,l-neomenthol and d,l-isomenthol in a weight ratio of about 6 : 3 : 1, corresponding to a state of equilibrium, is generally obtained, irrespective of the particular starting material, both in the case of catalytic hydrogenation and in the case of racemisation on account of the epimerisation which takes place in both processes. In addition to the two aforementioned stereoisomers, however, secondary products such as d,l-neoisomenthol, menthone and the stereoisomeric ketones, d,l-neomenthone and d,l-isomenthone are also formed together with menthenone and hydrocarbons (menthenes and menthane). The fact that it is difficult if not impossible to reduce the formation of these secondary products and/or to rearrange them into d,l-menthol, accounts for the considerable disadvantages of conventional processes. In the process according to U.S. Pat. Specification No. 2,843,636 for example, nonreuseable hydrocarbons are formed in a quantity of about 5%, whereas, as our own tests have shown, approximately 7% of undesirable secondary products are formed in the process according to U.S. Pat. Specification No. 2,871,272 which does not give any analytical data in respect of the resulting crude product.
These disadvantages become particularly noticeable when attempts are made to carry out the processes, described only in their batch form, in a continuous cycle by separating the d,l-menthol from the product stream by physical processes and recycling the secondary products. These undesirable secondary products accumulate so quickly in the recycle stream that continuous working becomes uneconomical.